1. Field of the Invention
The invention relates to the field of optical communication systems, and in particular, a transmitter configured to transmit an optical signal with corresponding Wavelength Division Multiplexing (WDM) and Time Division Multiplexing (TDM) channels.
2. Description of the Prior Art
In a fiber optic system, a transmitter transmits an optical signal over a fiber to a receiver. Designers of fiber optic systems work to increase transfer capacities of the system to accommodate more data traffic and the Internet. Transmitters use Wavelength Division Multiplexing (WDM) and Time Division Multiplexing (TDM) to increase the fiber optic system capacities.
A conventional transmitter is comprised of a laser and a modulator. The laser generates a laser signal and transfers the laser signal to the modulator. The modulator modulates the laser signal to represent data to be transmitted over an optical fiber. The modulator modulates either directly or externally. To directly modulate, the modulator varies an injected current supplied to the laser and changes the intensity of the laser signal. To externally modulate, the modulator receives the laser signal and an electric modulation signal. The electric modulation signal represents the data to be transmitted over the fiber. The modulator changes the amplitude and/or phase of the laser signal based on the electric modulation signal. An example of an external modulator is a Mach-Zehnder interferometer.
Time Division Multiplexing (TDM) transmits a number of signals simultaneously over a communications medium by interleaving sections of the individual data signals one after another. TDM divides a bandwidth of a TDM optical signal into fixed time slots of a fixed capacity, wherein the time slots represent time slot channels. To implement TDM in a fiber optic system, a time division multiplexer receives the individual data signals and assigns a time slot channel to the individual data signals. The time division multiplexer breaks the data signals into sections. The time division multiplexer plugs the sections of the data signals into the time slots of the TDM optical signal corresponding to the time slot channel assigned to the data signals.
In a TDM fiber optic system, nodes in the system are assigned time slot channels. Nodes drop the assigned time slot channels from the TDM optical signal using a demultiplexer, such as a Synchronous Optical NETwork (SONET) Add-Drop Multiplexer (ADM).
A problem with TDM is the fiber optic system uses multiple demultiplexers to drop the time slot channels from the TDM optical signal at the nodes. The separate nodes use separate demultiplexers to drop the time slot channels from the TDM optical signal. The number of demultiplexers depends on the number of nodes in the system. Demultiplexers are relatively expensive and increase the cost of the TDM fiber optic system.
Wavelength Division Multiplexing (WDM) passes multiple data channels over one or more wavelengths of light simultaneously over a single fiber. To implement WDM in a fiber optic system, multiple transmitters transmit optical signals over separate wavelength channels to a multiplexer. The multiplexer combines the optical signals to produce a WDM optical signal comprised of multiple wavelength channels. For example, a first transmitter transmits a first optical signal over a first wavelength channel, a second transmitter transmits a second optical signal over a second wavelength channel, and a third transmitter transmits a third optical signal over a third wavelength channel. A multiplexer receives and multiplexes the first, second, and third optical signals to produce the WDM optical signal comprised of a first wavelength channel, a second wavelength channel, and a third wavelength channel.
In a WDM fiber optic system, nodes in the system are assigned wavelength channels. Nodes drop the assigned wavelength channels from the WDM optical signal using an Optical Add-Drop Multiplexer (O-ADM). One example of the O-ADM is a conventional filter. Nodes also add the wavelength channels back to the WDM optical signal.
A problem with WDM is the fiber optic system uses multiple transmitters to transmit the WDM optical signal. WDM uses separate transmitters for the separate wavelength channels in the WDM optical signal. As the number of wavelength channels in the WDM optical signal increase, the number of transmitters increases raising the cost of the fiber optic system.